CDMA is a signal modulation technique used in a variety of applications, such as cellular and wireless communications systems. In such systems, multiple users communicate at will over a common frequency band with a base station. Each of these users transmits a uniquely ceded signal. Therefore, the received signal at the base station is a composite of many differently coded signals. Each coded signal is formed by multiplying each of a succession of digital symbols by more than one code coefficient. The product of each symbol times one code coefficient is a "chip" and each chip extends over a portion of the bit interval known as a chip interval. At the receiver, each user's encoded digital symbols are recovered from the incoming composite signal using code coefficients which replicate those used by each user.
It has long been recognized that during transmission a substantial amount of interference can be introduced into each coded signal from the other coded signals and compensation for this interference must be provided for intelligible communications. To reduce this interference, a number of different interference reduction techniques have been devised. In one prior art technique, apparatus is used in the receiver which operates on each user's symbols outputted by a despreader using priorly recovered other users' symbols. See, for example, U.S. Pat. No. 5,136,612, issued Aug. 4, 1992 and entitled "Method and Apparatus for Reducing Effects of Multiple Access Interference in a Radio Receiver in a Code Division Multiple Access Communication System." Another class of prior art systems uses an approach which operates on the received composite signal over a time interval using blocks of code coefficients wherein each block includes the code coefficients of each user corresponding to this time interval. See, for example, a publication entitled "Near-Far Resistance of Multiuser Detectors in Asynchronous Channels," I.E.E.E. Transactions on Communication, Vol. 38, No. 4, April 1990, and, more recently, a pending patent application entitled Data Recovery Technique for Asynchronous CDMA Systems, Ser. No. 07/982168, filed on Nov. 24, 1992, and assigned to the present assignee. Yet another class of prior an systems reduces interference by despreading the received signal using code coefficient sequences which are different from those utilized by the users to generate their respective coded signals. See, for example, pending U.S. application Ser. No. 07/991841, entitled "Despreading Technique for CDMA Systems," filed Dec. 16, 1992, and assigned to the present assignee. While all of these prior art arrangements substantially improve the performance of standard CDMA systems, there are applications where it is desirable to provide still higher performance levels without incurring any substantial additional costs.
It would, therefore, be desirable if a low-cost, data recovery technique could be developed for CDMA systems which provides a level of interference lower than that obtainable with prior art arrangements.